Method for producing phosgene

ABSTRACT

A method is disclosed for producing phosgene which in one embodiment comprises contacting in at least one reactor a mixture comprising carbon monoxide and chlorine sequentially with a first catalyst followed by contacting the resulting gaseous reaction mixture comprising phosgene with at least one second catalyst of higher relative activity than a first catalyst. In another embodiment a method is disclosed for producing phosgene which comprises contacting in at least one reactor a mixture comprising carbon monoxide and chlorine with at least one catalyst bed comprising a first catalyst wherein at least a portion of said first catalyst is diluted with a second catalyst of higher relative activity than a first catalyst.

BACKGROUND OF THE INVENTION

The present invention is directed to a method for producing phosgene.More particularly the invention relates to a method for producingphosgene by reaction of carbon monoxide and chlorine in the presence ofat least one catalyst.

Phosgene (sometimes known as carbonyl chloride or carbonyl dichloride)finds use in the preparation of organic compounds, monomers andpolymers, such as carbonates, isocyanates, chloroformates, carbamates,polyurethanes, and polycarbonates. Various methods for producingphosgene have been previously described in the literature and utilizedby industry. One method that is frequently used involves reaction ofcarbon monoxide with chlorine in the presence of a carbon-comprisingcatalyst such as activated carbon. The reaction is strongly exothermicand is usually done in multitubular reactors to more effectively controlthe reaction temperature. Phosgene produced by this method typicallycontains carbon tetrachloride as a byproduct, often as much as about250-300 ppm or higher by volume, which may be difficult to separate fromphosgene product. For some applications the level of carbontetrachloride in phosgene often must be removed to a level of about 1ppm before use of the phosgene. Carbon tetrachloride arises fromreaction of chlorine with carbon-comprising catalyst and thus resultsdirectly in depletion of catalyst. Carbon tetrachloride has also beenimplicated in both ozone depletion and in global warming. On a globalbasis the amount of byproduct carbon tetrachloride produced incommercial phosgene manufacture annually may be as much as 2 millionkilograms based on phosgene production of about 4 billion kilograms.Therefore, methods continue to be sought for minimization of carbontetrachloride byproduct formation using carbon catalysts whilemaintaining as high a rate of formation of phosgene as possible.

Japanese patent publication No. 02[1990]-06,307 discloses that theamount of carbon tetrachloride produced during a phosgene manufacturingprocess can be reduced by about 50% to about 150 ppm by using anactivated carbon which has been washed with an acid and which contains atotal of about 1.5 wt. % or less of metal components comprised oftransition metals, boron, aluminum and silicon.

SUMMARY OF THE INVENTION

After diligent experimentation the present inventors have discovered amethod for substantially reducing the amount of carbon tetrachloridebyproduct produced in phosgene manufacture while maintaining a high rateof formation of phosgene. In one of its embodiments the presentinvention is a method for producing phosgene which comprises contactingin at least one reactor a mixture comprising carbon monoxide andchlorine sequentially with a first catalyst followed by contacting theresulting gaseous reaction mixture comprising phosgene with at least onesecond catalyst of higher relative activity than a first catalyst. Inanother of its embodiments the present invention is a method forproducing phosgene which comprises contacting in at least one reactor amixture comprising carbon monoxide and chlorine with at least onecatalyst bed comprising a first catalyst wherein at least a portion ofsaid first catalyst is diluted with a second catalyst of higher relativeactivity than a first catalyst various other features, aspects, andadvantages of the present invention will become more apparent withreference to the following description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for producing phosgene in one embodiment ofthe invention.

FIG. 2 shows graphs of axial centerline temperatures along the length ofa reaction tube for a phosgene synthesis reactions using differentcatalysts all run at 300° C. reactor set temperature and at a combinedflow rate of carbon monoxide and chlorine of 250 standard cubiccentimeters per minute.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment the present invention is a method for producingphosgene containing very low levels of carbon tetrachloride byproduct.Phosgene made using the methods of the present invention contains in oneembodiment less than about 100 ppm by volume of carbon tetrachloride, inanother embodiment less than about 55 ppm by volume of carbontetrachloride, in still another embodiment less than about 20 ppm byvolume of carbon tetrachloride, in still another embodiment less thanabout 10 ppm by volume of carbon tetrachloride, in still anotherembodiment less than about 7 ppm by volume of carbon tetrachloride, instill another embodiment less than about 5 ppm by volume of carbontetrachloride, and in still another embodiment less than about 1 ppm byvolume of carbon tetrachloride.

Carbon monoxide and chlorine reaction gases employed in the preparationof phosgene are typically high purity grades, although a suitable carbonmonoxide is often supplied from an on-site generating plant and maycomprise trace amounts of impurities, such as, but not limited to,hydrogen, methane, volatile sulfur compounds, and nitrogen. Recycledcarbon monoxide recovered from phosgene product stream may also beemployed as part of the carbon monoxide-comprising feed stream. In oneembodiment essentially pure carbon monoxide and essentially purechlorine are used.

The compositions of mixtures of carbon monoxide and chlorine can bevaried in concentration ranges. In one embodiment carbon monoxide isused in equimolar amount or in molar excess of chlorine. In theseembodiments there is typically employed a carbon monoxide : chlorinemolar ratio (normalized on chlorine) in a range of between about 1.00:1and about 1.25:1, in another embodiment in a range of between about1.01:1 and about 1.20:1, in still another embodiment in a range ofbetween about 1.01:1 and about 1.12:1, in still another embodiment in arange of between about 1.02:1 and about 1.12:1, and in yet still anotherembodiment in a range of between about 1.02:1 and about 1.06:1. Inanother embodiment it is desirable to convert as much of the chlorinefeed to phosgene as is possible and to minimize the amount of residualchlorine in the reaction product. This may be generally accomplished bymaintaining a stoichiometric excess of carbon monoxide in the reactor todrive the reaction towards completion with respect to chlorine.

In one embodiment all the carbon monoxide requirement and all thechlorine requirement are fed initially to a reactor. In anotherembodiment at least some but less than all the requirement of carbonmonoxide is introduced to a first stage reaction zone with the remainingcarbon monoxide being introduced to at least one downstream reactionzone, wherein said at least one downstream reaction zone is in serialcommunicating relationship with said first reaction zone. In this latterembodiment an initial molar ratio of carbon monoxide: chlorine is lessthan one, and in certain embodiments the initial molar ratio of carbonmonoxide: chlorine (normalized on chlorine) is in a range of betweenabout 0.999:1 and about 0.2:1; in other embodiments is in a range ofbetween about 0.999:1 and about 0.5:1; in other embodiments is in arange of between about 0.999:1 and about 0.8:1; in other embodiments isin a range of between about 0.999:1 and about 0.95:1; and in still otherembodiments is in a range of between about 0.999:1 and about 0.98:1.

In one of its embodiments the present invention is a method forproducing phosgene which comprises contacting in at least one reactor amixture comprising carbon monoxide and chlorine sequentially with afirst catalyst followed by contacting the resulting reaction mixturecomprising phosgene with at least one second catalyst of higher relativeactivity than a first catalyst. In a particular embodiment any catalystafter a first catalyst has a higher relative activity for phosgeneformation than a first catalyst. In another particular embodiment thepresent invention is a method for producing phosgene which comprisescontacting in at least one reactor a mixture comprising carbon monoxideand chlorine sequentially with at least two catalysts, wherein anycatalyst after a first catalyst has a higher relative activity forphosgene formation than the catalyst immediately preceding it. Relativeactivity in this context may be measured by methods known in the art. Inone embodiment relative activity may be measured by comparing theexotherm associated with reaction of carbon monoxide and chlorine incontact with each catalyst separately under similar conditions, with ahigher exotherm indicative of a more active catalyst. In anotherembodiment relative activity may be measured by comparing the onset ofchlorine breakthrough as a function of flow rate and reactor settemperature in the reaction of carbon monoxide and chlorine at aspecific molar ratio in contact with each catalyst separately undersimilar conditions, with a breakthrough at a lower flow rate at a givenreactor set temperature or breakthrough at a lower reactor settemperature at a given flow rate indicative of a less active catalyst.In the present context similar conditions may include similar ratios ofcarbon monoxide to chlorine, similar amounts of catalyst, similar flowrates, similar reactors, and similar reactor set temperatures. In stillanother embodiment relative activity may be measured by comparing therate constants for phosgene formation over each of two differentcatalysts, wherein the rate constant may be represented by equation (1):

d[COCl₂]/dt=k₀e^(−Ea/RT)[CO]([Cl₂]/A*[CO]+[COCl₂]^(m)  (1)

where k₀ is the rate constant, A is a constant, m=0.25, as given by E.N. Shapatina, V. L. Kuchaev, B. E. Penkovoy, and M. I. Temkin, inKinetics and Catalysis (Russian), 1976, volume 27, p. 644.

The relative amounts of first and second (and any subsequent) catalystsare such that phosgene is produced which contains an amount of carbontetrachloride which is less than that produced using higher activitycatalyst alone under essentially the same reaction conditions. In oneembodiment the amount by volume of lower activity catalyst is in a rangeof between about 0.5% and about 70% of the total volume of catalyst; inanother embodiment in a range of between about 1% and about 60% of thetotal volume of catalyst; in another embodiment in a range of betweenabout 2% and about 50% of the total volume of catalyst; in anotherembodiment in a range of between about 5% and about 50% of the totalvolume of catalyst; in another embodiment in a range of between about10% and about 30% of the total volume of catalyst; and in still anotherembodiment in a range of between about 10% and about 20% of the totalvolume of catalyst.

In one embodiment a first catalyst has a high selectivity for phosgeneformation measured using the same reaction conditions under which both afirst and a second (and any subsequent) catalyst are employed in anembodiment of the present invention. In the present context a highselectivity for phosgene formation includes the requirement thatphosgene may be produced which contains an amount of carbontetrachloride in a range of between 0 ppm and about 200 ppm by volumeusing the first catalyst alone. A first catalyst also has a higherrelative selectivity for phosgene than a second (and any subsequent)catalyst. In one embodiment relative selectivity may be measured by theamount of carbon tetrachloride byproduct associated with reaction ofcarbon monoxide and chlorine in contact with each catalyst separatelyunder similar conditions, with a higher carbon tetrachloride levelindicative of a less selective catalyst. In the present context similarconditions may include similar ratios of carbon monoxide to chlorine,similar amounts of catalyst, similar flow rates, similar reactors, andsimilar reactor set temperatures.

In another embodiment a catalyst bed comprises at least some portion offirst catalyst diluted with a second catalyst of higher relativeactivity than a first catalyst. Dilution may be essentially homogeneousor in a gradient or a combination of homogenous and gradient. In oneembodiment a first catalyst zone at the initial point of contact withreactant gases contains a first catalyst diluted with a minor proportionof a second catalyst of higher relative activity, while a secondcatalyst zone contains said second catalyst which is diluted with aminor proportion of said first catalyst. In another embodiment a secondcatalyst of relatively higher activity may be distributed in a gradientin a first catalyst with the lowest concentration of second catalystbeing present at the initial point of contact with reactant gases andthe concentration of second catalyst gradually increasing until thehighest concentration of second catalyst is attained at the exit pointof product gases from a catalyst bed. In another embodiment a secondcatalyst of relatively higher activity may be distributed in a gradientin a first catalyst within a first catalyst zone with the lowestconcentration of second catalyst being present at the beginning of afirst catalyst zone and the concentration of second catalyst graduallyincreasing until the highest concentration of second catalyst isattained at an end of a first catalyst zone, and a second catalyst iscontained within a second catalyst zone with essentially no firstcatalyst in admixture. Those skilled in the art will realize that thedistribution of any catalyst in any other catalyst may be essentiallyhomogeneous or in a gradient or somewhere in between without departingfrom the embodiments of the present invention. In some embodiments atthe initial point of contact of catalyst with reactant gases aproportion of first catalyst may be present undiluted with secondcatalyst of higher relative activity, while any remaining portion offirst catalyst may be diluted with second catalyst of higher relativeactivity.

In another of its embodiments the present invention is a method forproducing phosgene which comprises contacting in at least one reactor amixture comprising carbon monoxide and chlorine sequentially with atleast one first catalyst zone comprising a catalyst following bycontacting the resulting mixture comprising phosgene with at least onesecond catalyst zone comprising said catalyst with a higher totalsurface area than that catalyst in a first catalyst zone. In thisembodiment essentially a single type of catalyst is employed. A firstcatalyst zone is at the initial point of contact with reactant gases. Ahigher total surface area of catalyst in a second (and any subsequent)catalyst zone compared to catalyst in a first catalyst zone containingessentially the same catalyst may be achieved by any convenient method.In one embodiment a first catalyst zone contains catalyst diluted withan inert filler which does not itself react under the reactionconditions and which does not catalyze or otherwise inhibit the phosgenesynthesis reaction, while a second catalyst zone contains the samecatalyst which is diluted with less inert filler than that in a firstcatalyst zone. In another embodiment a first catalyst zone containscatalyst diluted with an inert filler while a second catalyst zonecontains the same catalyst which is not diluted with inert filler. Inertfiller may be essentially evenly distributed among catalyst particlesand two catalyst zones may be sequentially loaded with catalystcontaining inert filler in a first catalyst zone followed by catalyst ina second catalyst zone containing less inert filler. Alternatively,inert filler may be distributed in a gradient among catalyst particlesin each catalyst zone with the highest concentration of inert fillerbeing present at the beginning of a first catalyst zone and theconcentration of inert filler gradually decreasing until the lowestconcentration of inert filler is attained at an end of a second catalystzone. In another embodiment inert filler may be distributed in agradient among catalyst particles in a first catalyst zone with thehighest concentration of inert filler being present at the beginning ofa first catalyst zone and the concentration of inert filler graduallydecreasing until the lowest concentration of inert filler is attained atan end of a first catalyst zone, and the second catalyst zone containsno inert filler. Those skilled in the art will realize that thedistribution of any filler in any catalyst zone may be essentiallyhomogeneous or in a gradient or somewhere in between without departingfrom the scope of the present invention. In some embodiments aproportion of catalyst near tie exit point of product gases from acatalyst bed may be present undiluted with inert filler, while anyremaining portion of catalyst nearer the initial point of contact ofcatalyst with reactant gases may be diluted with inert filler.

The types of inert filler are not particularly limited. In oneembodiment inert fillers are low porosity materials. In anotherembodiment inert fillers may be selected from the group consisting ofceramics, graphite, glassy carbon, glass, quartz, and metals. Suitablemetals comprise those which are not reactive under the reactionconditions and more particularly which are not reactive toward chlorine,carbon Monoxide, or phosgene under the reaction conditions . In oneembodiment metals suitable for inert fillers are selected from the groupconsisting of stainless steel, titanium, nickel, or metal alloys,including, but not limited to, nickel alloys comprising iron andchromium (such as INCONEL), or nickel alloys comprising molybdenum andchromium (such as HASTELLOY). In another embodiment suitable fillers areat least substantially inert in that they do not themselves react at anappreciable rate under the reaction conditions and do not catalyze orotherwise inhibit the phosgene synthesis reaction. Substantially inertin the present context means that a filler does not produce a level ofbyproducts that is outside a specification range for phosgene product.

In one embodiment the process of contacting in at least one reactor amixture comprising carbon monoxide and chlorine sequentially with atleast one first catalyst zone comprising a catalyst following bycontacting the resulting mixture comprising phosgene with at least onesecond catalyst zone comprising said catalyst with a higher totalsurface area than essentially the same catalyst in a first catalyst zoneis equivalent to providing a second catalyst of higher relative activitythan a first catalyst, wherein relative activity is measured asdescribed hereinabove.

In one embodiment at least one catalyst used in the methods of thepresent invention is a carbon-comprising catalyst, such as carbon. Anycarbon-comprising catalyst known in the art as a useful catalyst forphosgene preparation from carbon monoxide and chlorine may be used as atleast one catalyst in the method of the invention. In variousembodiments carbon from any of the following sources is useful as atleast one catalyst for the method of the invention; wood, peat, coal,coconut shells, bones, lignite, petroleum-based residues and sugar. Thecarbon can be in convenient forms such as powder, granules, particles,or pellets, or the like. In one embodiment the carbon surface area asdetermined by BET measurement is greater than about 100 square metersper gram (m²/g); in another embodiment greater than about 300 m²/g. Inanother embodiment active charcoal having a specific surface area ofabout 1,000 m²/g and having a particle size in a range of between about0.4 millimeters (mm) and about 5 mm may be employed. In still anotherembodiment carbon surface areas are in a range of between about 550 m²/gand about 1000 m²/g. In yet still another embodiment carbon surfaceareas are about 2000 m²/g or less.

In one embodiment there is employed as at least one catalyst a carbonwhich (1) has an active metal content of less than about 1000 ppm byweight, and (2) loses about 12% of its weight, or less, whensequentially heated in air for the following times and temperatures;125° C. for 30 minutes, 200° C. for 30 minutes, 300° C. for 30 minutes,350° C. for 45 minutes, 400° C. for 45 minutes, 450° C. for 45minutesand finally at 500° C. for 30 minutes as disclosed in U.S. Pat. No.6,022,993. This sequence of time and temperature conditions forevaluating the effect of heating carbon samples in air may be ran usingthermal gravimetric analysis (TGA). Carbons which when subjected to thisprotocol lose about 12% of their weight, or less, are considered to beadvantageously oxidatively stable.

As used in the context of this embodiment the term “active metals”includes transition metals of Groups 3 to 10 of the Periodic Table,boron, aluminum and silicon. Carbon which contains less than about 1000ppm by weight of active metals is typically employed in this embodiment.Iron is considered a particularly harmful active metal (i.e., thegreater the amount of iron the larger the amount of carbon tetrachlorideproduced). In a particular embodiment carbons are used which not onlyhave an active metal content of less than about 1000 ppm by weight, butalso contain less than about 100 ppm by weight of iron. In anotherparticular embodiment carbons are used which not only have an activemetal content of less than about 1000 ppm by weight, but also containless than about 80 ppm by weight of iron. In another particularembodiment carbons are used which contain less than about 200 ppm byweight of sulfur and less than about 200 ppm by weight of phosphorus. Inanother particular embodiment carbons are used which contain less thanabout 100 ppm by weight of sulfur and less than about 100 ppm by weightof phosphorus. Commercially available carbons which are included inthese particular embodiments include those sold under the followingtrademarks: Barnebey Sutcliffe™, Darco™, Nuchar™, Columbia JXN™,Columbia LCK™, Calgon PCB™, Calgon BPL™, Westvaco™, Norit™ and BarnebeyCheny NB™. In another embodiment acid-washed carbons are employed (forexample, carbons which have been treated with hydrochloric acid orhydrochloric acid followed by hydrofluoric acid). Acid treatment isoften sufficient to provide carbons which contain less than about 1000ppm of active metals. Suitable acid treatment of carbons is describedfor example in U.S. Pat. No. 5,136,113.

In another embodiment at least one catalyst used in the method of thepresent invention is a three dimensional matrix, porous carbonaceousmaterial. Examples include those described in U.S. Pat. No. 4,978,649.Of note are three dimensional matrix carbonaceous materials which areobtained by introducing gaseous or vaporous carbon-containing compounds(for example, hydrocarbons) into a mass of granules of a carbonaceousmaterial (for example, carbon black); decomposing the carbon-containingcompounds to deposit carbon on the surface of the granules; and treatingthe resulting material with an activator gas comprising steam to providea porous carbonaceous material. A carbon-carbon composite material isthus formed which is suitable as a catalyst.

In another embodiment there is employed as at least one catalyst acarbon having properties which include (1) a micropore to macroporeratio of 3.5 or less; and (2) loss of about 16% of its weight or less,in another embodiment about 10% of its weight or less, and in stillanother embodiment about 5% of its weight or less when sequentiallyheated in air for the following times and temperatures: 125° C. for 30minutes, 200° C. for 30 minutes, 300° C. for 30 minutes, 350° C. for 45minutes 400° C. for 45 minutes, 450° C. for 45 minutes and finally at500° C. for 30 minutes as disclosed in U.S. Pat. No. 6,054,612. In thisembodiment the active metal content of the carbon may be 1000 ppm ormore. The sequence of time and temperature conditions for evaluating theeffect of heating carbon samples in air may be run using thermalgravimetric analysis (TGA).

The carbon materials in this embodiment are porous (i.e., they possess asurface area of at least 10 m²/g), and contain both micropores andmacropores. As used in this context, the term “micropore” means a poresize of about 20 Å (angstroms) or less and the term “macropore” means apore size of greater than about 20 Å. The total pore volume and the porevolume distribution can be determined, for example, by methods known inthe art, such as by porosimetry. The micropore volume (cc/g) issubtracted from the total pore volume (cc/g) to determine the macroporevolume. The ratio of micropores to macropores is then easily calculated.The carbons materials in one embodiment have a micropore to macroporeratio of less than about 3.5, in another embodiment about 2.0 or less,and in still another embodiment about 1.0 or less. In yet still anotherembodiment the carbons materials have a micropore to macropore ratio ofabout zero. Commercially available carbons which may be used in theseparticular embodiments include those sold under the trademarks CalgonX-BCP™ and Calsicat™.

In another embodiment there is employed as at least one catalyst acarbon having properties which include (1) a pore volume in a range ofbetween about 0.2 cubic centimeters per gram (cc/g) and about 1.7 cc/gformed by bent layers of carbon of a thickness in a range of betweenabout 100 angstroms (Å) and about 10,000 Å and a radius of curvature ina range of between about 100 Å and about 10,000 Å, (2) a true density ina range of between about 1.80 grams per cubic centimeter (g/cc) andabout 2.10 g/cc, (3) an X-ray density in a range of between about 2.112g/cc and about 2.236 g/cc, and (4) a pore size distribution having itsmaximum within the range of between about 200 Å and about 2000 Å asdisclosed in U.S. Pat. No. 4,978,649.

In another embodiment a silicon carbide-containing catalyst with asurface area greater than about 10 square meters per gram may be used asat least one of the catalysts in the process of the present invention.In another embodiment a silicon carbide-containing catalyst with asurface area greater than about 20 square meters per gram may be used asat least one of the catalysts in the process of the present invention.In still another embodiment a silicon carbide having a surface areagreater than about 100 m²/g, prepared by methods disclosed in U.S. Pat.No. 4,914,070 may be used as at least one of the catalysts. In oneembodiment a silicon content of at least about 5 weight % is used. Inanother embodiment a silicon content is at least about 10 weight % isused. Of note are embodiments where the catalyst is manufactured using aprocess which comprises contacting silicon monoxide with finely dividedcarbon as disclosed, for example, in U.S. Pat. No. 4,914,070. In oneembodiment a carbon which has an ash content of less than about 0.1 wt.% is used to produce the silicon carbide.

In one embodiment a silicon carbide catalyst suitable for use as atleast one catalyst in the present invention is prepared by a processdisclosed in U.S. Pat. No. 6,054,107 which comprises reacting vapors ofsilicon monoxide, SiO, on carbon by the steps of: (a) generating vaporsof SiO in a first reaction zone by heating a mixture of SiO₂ and Si at atemperature of between about 1100° C. and about 1400° C., under apressure in a range of between about 0.1 kilopascals and about 1.5kilopascals; and (b) contacting in a second reaction zone at atemperature in a range of between about 1100° C. and about 1400° C., theSiO vapors generated in said first reaction zone with finely dividedreactive carbon with a specific surface area that is equal to or greaterthan about 200 m2/g. Examples of suitable reactive carbons includegraphite pellets obtained by powder agglomeration, and activated carbonsuch as powdered activated carbon obtained by crushing granules ofactivated carbon. In one embodiment a silicon carbide surface area asdetermined by BET measurement is greater than about 100 m²/g and inanother embodiment is greater than about 300 m²/g.

First and second (and any subsequent) catalysts, or in anotherembodiment, first and second (and any subsequent) catalyst zones, may becontained in a single reactor or in an alternative embodiment in two ormore separate reactors which may be in serial communicating relationshipwith each other. Typically a reactor comprises at least one reactionvessel to contain catalyst. For example, a first catalyst (or firstcatalyst zone) may be contained in a first reactor (sometimes referredto in the art as a primary reactor) and a second catalyst (or secondcatalyst zone) of higher relative activity than first catalyst may becontained in a second reactor (sometimes referred to as a finishingreactor or downstream reactor). In another embodiment at least onereactor may be loaded sequentially with a volume of first catalystfollowed by a volume of second catalyst. The types of reactor orreactors include those known in the art, illustrative examples of whichinclude shell-and-tube-type reactors, jacketed tank-type of reactors,and conventional water- or low pressure steam-jacketed tank-type ofreactors.

In another embodiment first and second (and any subsequent) catalysts,or in another embodiment, first and second (and any subsequent) catalystzones are contained in at least one reaction tube such as is illustratedin FIG. 1 which shows an apparatus 10 for producing phosgene in oneembodiment of the invention. As shown in FIG. 1 a suitable reactor maybe a tubular flow reactor comprising at least one reaction tube 12.However, the reactor may also be designed in a different way. In theembodiment illustrated in FIG. 1 a reaction tube 12 contains an optionalslide tube 14 smaller in diameter than reaction tube 12 for housing anoptional movable thermocouple 16. Optional slide tube 14 extends theentire length of tube 12 and is welded closed at its lower end. Thereaction tube 12 is fitted with a porous frit 26 attached at its lowerend to serve as a pedestal fitting snugly into the inner reaction tube12 diameter. This pedestal acts as a support for a catalyst bed 18 thatis added to the reactor. The end of optional slide tube rests 14 on theporous frit 26 ensuring that the bottom of a catalyst bed 18 coincideswith the furthermost location of the optional movable thermocouple 16 inthe optional slide tube 14. By changing the position of the movablethermocouple 16 in the slide tube 14, the temperature along the lengthof the reaction tube 12 and particularly in the catalyst bed 18 may bemeasured. A porous, inert heat transfer medium 28 of high surface areasuch as a wound fine meshed metal screen is positioned at the entranceof the reaction tube 12 so that the reactor is in plug flow regime andthe feed gases entering the entrance 30 of the reaction tube 12 attainthe approximate pre-set reaction temperature before contact with thecatalyst bed 18. The catalyst bed 18 comprises a lower activity catalyst22 and a relatively higher activity catalyst 20. A lower activitycatalyst 22 is contained in the bottom part of reaction tube 12 and isseparated from a relatively more active catalyst 20 above it using aporous separation grid 24. The porous separation grid 24 essentiallykeeps a lower activity catalyst 22 and a relatively higher activitycatalyst 20 from intermixing. In alternative embodiments a porousseparation grid 24 is not present. The reaction tube 12 is in contactwith a solid heat transfer medium 34 such as, but not limited to,aluminum, which is in turn in contact with a heat source 32, such as afurnace.

A reaction tube 12 may be made from a corrosion resistant material ormay be lined with a corrosion resistant material. In the present contexta corrosion resistant material is one which is essentially inert tochlorine, carbon monoxide, and phosgene. Illustrative corrosionresistant materials comprise ceramic, stainless steel, titanium, nickel,or metal alloys, including, but not limited to, nickel alloys comprisingiron and chromium (such as INCONEL), or nickel alloys comprisingmolybdenum and chromium (such as HASTELLOY). Porous frit 26, porousseparation grid 24, and optional slide tube 14 may be made from similaror different corrosion resistant materials.

Reaction temperature in any apparatus used to make phosgene by themethod of the present invention may be varied in a broad range and isgenerally chosen in consideration of such factors as maximization ofphosgene space-time yield and minimization of chlorine breakthrough andcarbon tetrachloride production. In one embodiment a reactor may be runat the highest possible temperature that achieves maximum production ofphosgene without chlorine breakthrough. In another embodiment reactiontemperature is measured in the reactant gas inlet stream into a reactorand the product gas outlet stream from a reactor, and the temperaturesare in a range of between about 30° C. and about 350° C.; in anotherembodiment in a range of between about 30° C. and about 300° C.; instill another embodiment in a range of between about 30° C. and about200° C.; in still another embodiment in a range of between about 30° C.and about 120° C.; and in yet still another embodiment in a range ofbetween about 30° C. and about 90° C.

In another embodiment a reaction temperature may be conveniently equatedwith an approximate reactor set temperature implemented through a sourceof heat such as a furnace (32 of FIG. 1) and measured by one or morethermocouples positioned in solid heat transfer medium 34 of FIG. 1.Reactor set temperatures are in one embodiment in a range of betweenabout 30° C. and about 350° C.; in another embodiment in a range ofbetween about 80° C. and about 300° C.; and in still another embodimentin a range of between about 100° C. and about 200° C.

Without further elaboration, it is believed that one skilled in the artcan, using the description herein, utilize the present invention to itsfullest extent. The following examples are included to provideadditional guidance to those skilled in the art in practicing theclaimed invention. While some of the examples are illustrative ofvarious embodiments of the claimed invention, others are comparative.The examples provided are merely representative of the work thatcontributes to the teaching of the present application. Accordingly,these examples are not intended to limit the invention, as defined inthe appended claims, in any manner.

Experiments were carried out in a tubular flow reactor as shown in FIG.1. The reactor consisted of a reaction tube (12) about 2 feet long and0.5 inch outside diameter (OD) into which catalyst could be poured andretained. The reaction tube (12) was fitted with a small diameter (0.125inches) axial slide tube (14) extending the entire length of thereaction tube 12 to hold a movable thermocouple 16. The slide tube 14was welded closed at its lower end. The reaction tube 12 was fitted witha porous frit 26 attached at its lower end to serve as a pedestal thatfit snugly into the inner reaction tube diameter. This pedestal acted assupport for the catalyst bed 18 that was added to the reactor. The endof the slide tube 14 rested on the pedestal ensuring that the bottom ofthe catalyst bed 18 coincided with the furthermost location of themovable thermocouple 16 in the slide tube 14. A fine meshed, corrosionresistant metal screen (28) about 20 centimeters (cm) to about 25 cmlong was tightly wound and pushed into the entrance of the reactor. Itensured that the entrance to the catalyst bed was in plug flow regimeand allowed the feed gases to attain the approximate pre-set reactortemperature before contact with the catalyst bed 18. All parts of thereactor contacting with chlorine including the screen 28 and theseparation grid 24 were constructed of INCONEL 600 alloy.

Two different catalysts were employed. The first, referred tohereinafter as “Catalyst A” and described in U.S. Pat. No. 4,978,649,was a porous carbonaceous material in the form of a three-dimensionalmatrix with a pore volume in a range of between about 0.2 cubiccentimeters per gram (cc/g) and about 1.7 cc/g formed by bent layers ofcarbon of a thickness in a range of between about 100 angstroms (Å) andabout 10,000 Å and a radius of curvature in a range of between about 100Å and about 10,000 Å, a true density in a range of between about 1.80grams per cubic centimeter (g/cc) and about 2.10 g/cc, an X-ray densityin a range of between about 2.112 g/cc and about 2.236 g/cc, and a poresize distribution having its maximum within the range of between about200 Å and about 2000 Å. A second catalyst employed was Barnebey Cheneycoconut shell carbon referred to hereinafter as “Catalyst B”. Catalyst Bhad a higher activity than Catalyst A.

The lower activity catalyst, Catalyst A, was loaded into the bottom partof the reactor and separated from the more active catalyst above it,Catalyst B, with a separation grid 24. Two separate composite catalystbeds with 15% and 30% of Catalyst A by volume (with the remainder of thecatalyst bed volume comprising Catalyst B) were tested in comparisonwith a uniform catalyst bed of pure Catalyst B. The total length of eachcatalyst bed was 10 cm. Chlorine and carbon monoxide gases were made toflow over and through the catalyst bed. The carbon monoxide:chlorinemolar ratio was set in a range of between about 1.02 and about 1.1, andthe total gas flow was set in a range of between about 100 standardcubic centimeters per minute (sccm) and about 250 sccm using twomass-flow controllers. Total gas pressure was maintained at about 138kilopascals. Reactor temperature used was set in a range of betweenabout 100° C. and about 300 ° C. Axial centerline temperatures wereregistered by the movable thermocouple 16. Temperatures were measuredand analytical data collected after the reaction had reached a steadystate as measured by the stability of the temperature readings andanalysis of the gas mixture. The data reported in Table 1 are averagevalues for data points collected about every 30 minutes after thereaction reached steady state. The product gas mixture was analyzed bygas chromatography/mass spectroscopy (GC/MS). Chlorine breakthroughmeans that chlorine was detected. Values for carbon tetrachloride are inppm by volume. The results are shown in Table 1.

TABLE 1 Maximum Total flow, CO:Cl₂ axial center- Chlorine CCl₄, Catalystbed Set T, ° C. sccm ratio line T, ° C. breakthrough ppm* Catalyst B100% 100 100 1.1 166 no 5.9 100 250 1.1 260 no 5.6 100 250 1.02 261 no14.1 300 250 1.1 401 no 49.9 300 250 1.02 405 yes 71.2 Catalyst A 15%,100 100 1.1 153 no 2.3 Catalyst B 85% 100 250 1.1 216 no 4.3 100 2501.02 218 no 5.1 300 250 1.1 382 no 10.7 300 250 1.02 385 no 7.2 CatalystA 30%, 100 100 1.1 146 no 2.6 Catalyst B 70% 100 250 1.1 193 no 2.3 100250 1.02 198 no 4.5 300 250 1.1 380 no 8.7 300 250 1.02 384 no 16.4Catalyst A 100% 100 100 1.1 144 no 0.23 100 250 1.1 203 yes n/a 160 2501.1 270 yes 2.4 300 250 1.1 378 no 4.1 300 250 1.02 380 no 3.7 *Averageof 2-4 measurements

FIG. 2 shows graphs of axial temperature along the length of a reactiontube for a phosgene synthesis reactions using the different catalystsshown in Table 1 all run at 300° C. set temperature and at a combinedflow rate of carbon momonoxide and chlorine of 250 standard cubiccentimeters per minute.

FIG. 2 show that the use of composite catalyst beds comprisingrelatively low activity and relatively high activity catalyst noticeablydecreased the exotherm in the reactor hot zone compared to thatgenerated by the highly active carbon catalyst alone. In the case ofhigh set temperature (300° C.; FIG. 2), the exotherm decreased by 20° C.from the uniform bed of catalyst B to a composite catalyst bed. Therewas practically no difference between the 15% and 30% beds.

Using the composite catalyst beds, the level of carbon tetrachloride inphosgene stream decreased by factor of about two at low temperature andwas about 5 times lower at high temperature in comparison with theuniform Catalyst B bed. The carbon tetrachloride levels for thecomposite catalysts were still higher than that for pure Catalyst A, butthe activity of the composite beds was higher. No chlorine breakthroughwas observed for the composite beds even at high flow and temperature.In contrast, pure Catalyst A had chlorine breakthrough at high flow evenat the lower set temperature of 160° C. On the other hand, pure CatalystB showed chlorine breakthrough only at high flow and high temperaturewhen the carbon monoxide:chlorine ratio was reduced to 1.02.

These results demonstrate the beneficial use of the composite catalystbeds for phosgene production with decreased levels of byproduct carbontetrachloride. The method also extends the lifetime of carbon-comprisingcatalysts used in the manufacture of phosgene. Although the invention isnot meant to be limited by any theory of operation it is believed thatthe lower levels of byproduct carbon tetrachloride obtained usingcomposite catalyst beds is due to the lower reaction exotherm obtainedcompared to the reaction exotherm obtained using a homogeneous catalystbed containing only the relatively higher activity catalyst of acomposite catalyst bed.

Phosgene produced by the methods of the invention may be used directlyor may be recovered from the product gases by known methods such as bypassing the gases through a cooling system where they are cooledsufficiently to cause the phosgene contained therein to condense out.The condensed phosgene may then be recovered as a liquid product,usually under pressure.

While the invention has been illustrated and described in typicalembodiments, it is not intended to be limited to the details shown,since various modifications and substitutions can be made withoutdeparting in any way from the spirit of the present invention. As such,further modifications and equivalents of the invention herein disclosedmay occur to persons skilled in the art using no more than routineexperimentation, and all such modifications and equivalents are believedto be within the spirit and scope of the invention as defined by thefollowing claims. All U.S. Patents cited herein are incorporated hereinby reference.

What is claimed is:
 1. A method for producing phosgene which comprisescontacting in at least one reactor a mixture comprising carbon monoxideand chlorine sequentially with a first catalyst followed by contactingthe resulting gaseous reaction mixture comprising phosgene with at leastone second catalyst of higher relative activity than a first catalyst.2. The method of claim 1 wherein a first catalyst has a higher relativeselectivity for phosgene than a second catalyst, as measured by theamount of carbon tetrachloride associated with reaction of carbonmonoxide and chlorine in contact with each catalyst separately undersimilar conditions.
 3. The method of claim 1 wherein the carbonmonoxide:chlorine molar ratio is in a range of between about 1.01:1 andabout 1.20:1.
 4. The method of claim 1 wherein the carbonmonoxide:chlorine molar ratio is in a range of between about 1.02:1 andabout 1.12:1.
 5. The method of claim 1 wherein phosgene producedcontains less than about 55 ppm by volume of carbon tetrachloride. 6.The method of claim 1 wherein phosgene produced contains less than about20 ppm by volume of carbon tetrachloride.
 7. The method of claim 1wherein phosgene produced contains less than about 10 ppm by volume ofcarbon tetrachloride.
 8. The method of claim 1 wherein phosgene producedcontains less than about 7 ppm by volume of carbon tetrachloride.
 9. Themethod of claim 1 wherein phosgene produced contains less than about 5ppm by volume of carbon tetrachloride.
 10. The method of claim 1 whereinphosgene produced contains less than about 1 ppm by volume of carbontetrachloride.
 11. The method of claim 1 wherein at least one catalystcomprises carbon.
 12. The method of claim 1 wherein both a first and asecond catalyst comprise carbon.
 13. The method of claim 1 whichcomprises contacting in at least one reactor a mixture comprising carbonmonoxide and chlorine at a reaction temperature in a range of betweenabout 30° C. and about 350° C.
 14. The method of claim 1 which comprisescontacting in at least one reactor a mixture comprising carbon monoxideand chlorine at a reaction temperature in a range of between about 30°C. and about 120° C.
 15. The method of claim 1 which comprisescontacting in at least one reactor a mixture comprising carbon monoxideand chlorine at a reaction temperature in a range of between about 30°C. and about 90° C.
 16. The method of claim 1 which comprises contactingin at least one reactor a mixture comprising carbon monoxide andchlorine sequentially with at least one first catalyst zone comprising acatalyst followed by contacting the resulting mixture comprisingphosgene with at least one second catalyst zone comprising a catalystwith a higher total surface area than that catalyst in a first catalystzone.
 17. The method of claim 16 wherein at least one catalyst comprisescarbon.
 18. The method of claim 16 wherein catalysts in both a first anda second catalyst zone comprise carbon.
 19. The method of claim 18wherein catalysts in a first and second catalyst zone are derived fromthe same material.
 20. The method of claim 16 wherein catalyst in afirst catalyst zone is diluted with a filler inert to chlorine, carbonmonoxide, and phosgene.
 21. The method of claim 20 wherein the filler isdistributed in a gradient.
 22. The method of claim 20 wherein the filleris selected from the group consisting of ceramics, graphite, glassycarbon, glass, quartz, metals, stainless steel, titanium, nickel, andmetal alloys.
 23. The method of claim 22 wherein a metal alloy isselected from the group consisting nickel alloys comprising iron andchromium, and nickel alloys comprising molybdenum and chromium.
 24. Amethod for producing phosgene which comprises contacting chlorine and astoichiometric excess of carbon monoxide in at least one reactorsequentially with a first catalyst comprising a porous carbonaceousmaterial in the form of a three-dimensional matrix with a pore volume ina range of between about 0.2 cc/g and about 1.7 cc/g formed by bentlayers of carbon of a thickness in a range of between about 100 Å andabout 10,000 Å and a radius of curvature in a range of between about 100Å and about 10,000 Å, a true density in a range of between about 1.80g/cc and about 2.10 g/cc, an X-ray density in a range of between about2.112 g/cc and about 2.236 g/cc, and a pore size distribution having itsmaximum within the range of between about 200 Å and about 2000 Å,followed by contacting the resulting reaction mixture comprisingphosgene with a second catalyst comprising coconut shell carbon ofhigher relative activity than a first catalyst, wherein relativecatalyst activity is measured by the exotherm associated with reactionof carbon monoxide and chlorine in contact with each catalyst separatelyunder similar conditions.
 25. The method of claim 24 wherein the excessof carbon monoxide is in a range of between about 2% and about 12% ofsaid stoichiometric requirement.
 26. The method of claim 24 whereinphosgene produced contains less than about 20 ppm by volume of carbontetrachloride.
 27. The method of claim 24 wherein phosgene producedcontains less than about 10 ppm by volume of carbon tetrachloride. 28.The method of claim 24 wherein phosgene produced contains less thanabout 7 ppm by volume of carbon tetrachloride.
 29. The method of claim24 wherein phosgene produced contains less than about 5 ppm by volume ofcarbon tetrachloride.
 30. The method of claim 24 wherein phosgeneproduced contains less than about 1 ppm by volume of carbontetrachloride.
 31. The method of claim 24 wherein the amount of firstcatalyst is in a range between about 0.5% and about 70% of the totalcatalyst volume.
 32. A method for producing phosgene which comprisescontacting in at least one reactor a mixture comprising carbon monoxideand chlorine sequentially with at least two catalysts, wherein anycatalyst after a first catalyst has a higher relative activity forphosgene formation than the catalyst immediately preceding it.